1. Field of the Invention
The present invention relates to an image forming method for forming a multicolor or a full-color image with an electrophotographic process and an image forming apparatus using the same.
2. Description of the Background Art
One of conventional color image forming apparatuses is configured to sequentially transfer toner images of different colors from photoconductive drums or similar image carries to an intermediate image transfer body one above the other for thereby completing a multicolor toner image. The problem with this type of image forming apparatus is the reverse toner transfer to be described hereinafter. For example, in a tandem, image forming apparatus including a plurality of image forming sections arranged side by side and a single intermediate image transfer body, part of toner transferred to the intermediate image transfer body at an upstream image forming section is returned, or reversely transferred, to the image carrier of a downstream image forming section, giving rise to color mixture, image disturbance, smears and other defects. Such reverse toner transfer occurs when a bias for image transfer is applied at the downstream image forming section in order to transfer a toner image from the image carrier to the intermediate image transfer body, particularly in the non-image portions of the image carrier.
Reverse toner transfer is considered to derive from the following mechanism. The surface of the image carrier is charged to the same polarity as toner by charging means while the surface potential of the intermediate image transfer body is opposite in polarity to the toner or is 0 V. If a potential difference between the surface of the image carrier and that of the intermediate image transfer body is great, then the difference is particularly great in the non-image portions of the image carrier. In this condition, discharge occurs when the image carrier and intermediate image transfer body approach each other around a primary image transfer position, causing the resulting electronic ions to invert the charge polarity of part of the toner deposited on the intermediate image transfer body. Consequently, the toner thus inverted in polarity is subject to an electrostatic force acting toward the image carrier and is reversely transferred to the image carrier thereby.
Besides the electric influence stated above, reverse toner transfer is physically brought about by the influence of non-electrostatic adhering forces including an increase in toner adhering force that acts between the toner and the image carrier and toner adhering force that acts between them at the image transfer nip due to the pressure of an image transfer roller that presses the intermediate image transfer body against the image carrier. To obviate such a physical cause of reverse toner transfer, Japanese Patent Laid-Open Publication No. 2003-156947, for example, proposes to mount an abutment member on the shaft of a bias roller for primary image transfer, which faces an image carrier, via a bearing. The abutment member contacts the surface of the image carrier to thereby form a gap between the image carrier and an intermediate image transfer body. However, the resulting non-contact image transfer station obstructs faithful transfer of a toner image and brings about toner scattering.
On the other hand, Japanese Patent No. 3101276 discloses a color image recording apparatus including a pair of rollers held in contact with opposite end portions of a photoconductive drum in order to maintain the amount of nip between the drum and an intermediate image transfer body constant. Although such rollers maintain the nip width constant in contact with the photoconductive drum, they do not cope with reverse toner transfer at all.
Further, Japanese Patent Laid-Open Publication No. 9-305041 proposes to obviate reverse toner transfer by forming a gap between an image carrier and an image transfer roller and positioning the axis of the image transfer roller above the axis of the image carrier. This configuration, however, has problems left unsolved as to an increase in adhering force ascribable to the weight of a recording medium and the stability of image formation.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 06-102783, 10-198197 and 2001-066908.